Definition check: an ideal fluid is best described as which of the following?

Introduction / Context:
Many fluid mechanics derivations—Bernoulli’s equation, potential flow, and vortex theory—use the abstraction of an “ideal fluid.” Correctly recalling this definition is crucial when deciding whether a model applies to a real-world situation or only provides a first approximation.

Given Data / Assumptions:

• Ideal fluid is a theoretical construct.
• Used primarily for analytic tractability and conceptual understanding.
• Real fluids depart from this ideal to varying degrees.

Concept / Approach:

An ideal fluid has zero viscosity (inviscid) and is incompressible (density does not change with pressure). With no viscous stresses, shear stress is zero for any shear rate, so energy dissipation by viscosity is absent. Incompressibility eliminates density variations, simplifying continuity and momentum equations in many flows.

Step-by-Step Solution:

Verification / Alternative check:

Compare with Newtonian fluids (τ = μ du/dy). If μ ≠ 0, viscous effects exist; hence such a fluid is not ideal. Similarly, gases are compressible; liquids have finite viscosity—so neither are strictly ideal.

Why Other Options Are Wrong:

(b) is nonsensical; Newton’s law involves shear stress, not “velocity.” (c) Gases are typically compressible and viscous. (d) and (e) contradict inviscid, incompressible assumptions.

Common Pitfalls:

Over-applying ideal models to flows dominated by viscous effects or compressibility (e.g., near walls, high-speed gas flows).

Final Answer:

Frictionless (inviscid) and incompressible